Many blinding diseases involve loss of photoreceptor and retinal pigment epithelium (RPE) cells. Often, this cell death results from DNA aberrations, whether inherited or environmentally acquired. There are several biochemical and molecular processes for repairing DNA, in aggregate called the DNA Damage Response (DDR). The DDR is endogenously activated when damage is detected in genomic DNA. It can also be activated by experimentally inducing nonspecific or sequence-specific DNA damage. Unexpectedly, recent exciting developments show that the DDR can be activated even without DNA damage and that DDR activation with or without DNA damage can be a preconditioned, or protective, mechanism. Compared to non-differentiated cells, little is known about the DDR in terminally-differentiated cells such as those of the retina. This project is designed to increase our understanding of the endogenous DNA repair mechanisms of photoreceptor and RPE cells. By learning how DDR functions in retinal cells and how it can be activated experimentally, we can develop new strategies to enhance oligonucleotide-directed DNA repair, a gene therapy approach proven efficacious in retinal degeneration models in the previous funding period. Further, it may be that, analogous to hypoxic or cyclic light preconditioning, experimentally activating the DDR enhances the ability of retinal cells to withstand stresses that would otherwise lead to accumulation of DNA damage and death. Exploiting natural DNA repair processes thus could slow or prevent blindness by enhancing repair of specific gene mutations and by protecting retinal DNA from environmental insult. The specific aims of this project are to test the hypotheses that: 1) DDR in retinal cells can be experimentally- activated and can enhance oligonucleotide-directed gene repair; and 2) experimentally-activated DDR is protective against subsequent DNA damage or its effects.